On an unseasonably hot day in early June, Carole Price went shopping at Whole Foods in Austin, Texas, the flagship store. On this day, the lean 51-year-old unloaded half-and-half, sharp cheddar cheese, and yogurt, among assorted leafy vegetables and other goods from her cart. Each dairy product that rode the conveyor belt toward the beeping scanner was marked by the contrasting half-moons of the “USDA organic” label.

Carole Price always buys organic dairy. “I don’t want the antibiotics that they put in regular milk,” she says. “And I prefer to support a smaller-area industry than a big mega-farm.”

Her routine is becoming a familiar sight. More and more Americans are selecting organics to fill their cabinets and bellies. Last year, Americans spent nearly $25 billion on organic food—as much as the gross domestic product of the entire nation of Estonia. Our organic food spending has quadrupled in the 10 years since the word “organic” took on a legal meaning, and a lagging economy didn’t slow it down. In 2009, organic food sales grew by 5.1 percent, as compared with only 1.6 percent of overall food sales. Of all American consumers, three-quarters purchase organic food and beverages; over a third of them are over 45, according to a report about organic food by the Hartman Group. And organic food spending is projected to keep on growing.

But now, many meat and dairy producers must change the way they do business to earn the organic label. In February, the U.S. Department of Agriculture (USDA) finalized a new set of rules, which took effect in June. Producers seeking organic certification must prove that their ruminant animals, like cows, spend at least 120 days per year, or the duration of the grass-growing season, grazing on pasture. Moreover, grass must account for at least 30 percent of their food. Producers that were already certified organic before the rules’ release in February have an additional year to comply.

Why the change? The public perception of what organic means and the reality aren't necessarily the same. To most of us, the word “organic” conjures images of an idealized style of agriculture. The leaping cartoon cow on a half-gallon carton of organic milk evokes images of cattle grazing happily in green pastures, chewing on grass as nature intended.

But that isn’t necessarily what organic has meant in the United States. And to understand the new definition, you have to understand the old.

That’s not what I thought “organic” meant!

Since 2000, any food labeled organic must be produced according to a set of rules called the National Organic Program. These standards ensure, for example, that organic blueberries haven’t been treated with any of the chemicals on a long list the USDA maintains. They also specify that organic livestock never be treated with hormones or antibiotics or eat any feed that wasn’t itself organic, too.

But there’s nothing in the USDA standards that says organic lettuce can’t be grown by industrial-scale producers in giant greenhouses. (Only small farmers—those who sell $5,000 or less worth of agricultural products a year—are permitted to label food “organic” without certification under the national law.) The rules also haven’t prevented organic livestock from living in dusty, grassless pens as long as they have access to pasture.